Laboratory Testing of Milk Samples

For conducting some other tests to assess the freshness, hygiene and composition of milk, small laboratories and well trained technicians are needed. These tests are also needed to improve the quality of milk production and supply. Milk of initial good quality alone will give products of better quality. Moreover, the payment for milk is based on its constituents such as fat and solids-not-fat (SNF) contents. These tests are conducted in the laboratory.

 

i. Chemical Tests

Various chemical and physical tests conducted to assess the quality of milk received in a dairy include titratable acidity, fat content, total solids and solids-not-fat content,and tests for detection of adulterants.

Titratable Acidity: Acidity of milk is expressed in term of its lactic acid content.10 to 50 ml of milk sample is titrated with N/9 or N/10 NaOH solution and phenolphthalein as indicator. Normal acidity of fresh milk varies between 0.10 to 0.16 % lactic acid. The titratable acidity (T.A.) as lactic acid per 100 ml of milk is calculated by using formulae as under:

% Lactic acid = 9 * V1 * N1 / V2

where, V1 = Volume of standard NaOH solution, ml

N1 = Normality of standard NaOH solution

V2 = Volume of milk sample taken, ml

Fat Test: Fat test of milk is done for making payment of the milk. Among several methods, one common method is the acidobutyrometric butterfat test or Gerber test. Fat globule membranes and proteins of the milk are hydrolyzed with concentrated sulphuric acid to break the emulsion and to set the fat free. The volume of fat from a given quantity of milk sample is measured in a specially designed glass recipient, known as butyrometer. The butyrometer reading gives the result directly in fat percentage. For accurate results, the reading has to be taken quickly to avoid cooling of the fat column.

Now, Milko-tester Minor or semi-automatic butterfat analyzer is used for determining the fat content in milk. This is based on the principle of scattering of light by milk fat globules. Light transmission through the milk-mix is measured photometrically and read directly as the fat percentage in milk samples on digital display. Accuracy of the result is approximately the same as for the Gerber fat test.

Determination of total solids (TS) and Solids-not-fat (SNF) content: A lactometer is used to measure the specific gravity of milk. Based on the lactometer reading of milk taken under standard conditions and knowing the fat percentage as determined by the Gerber method, it is possible to calculate TS and SNF by using the well known Richmond’s formula which differs for the types of lactometer, viz. Quevenne, Zeal-and BIS-lactometer.

(i) Quevenne lactometer:

% TS = CLR/4 + 1.2 F + 0.14
% SNF= CLR/4+0.2 F + 0.14

(ii) Zeal lactometer

% TS = CLR/4 + 1.2 F + 0.50
% SNF= CLR/4+0.2 F + 0.50

(iii) BIS lactometer

% TS = CLR/4 + 1.2 F + 0.60
% SNF= CLR/4+0.2 F + 0.60

Where, CLR = corrected lactometer readings, obtained by applying the specific correction factor to the observed lactometer readings based on temperature of milk.

Determination of TS and SNF of milk can help to detect the adulteration of milk.TS and SNF also form the basis for the pricing of milk.

 

ii. Microbiological Tests

Microbiological tests provide information on the sanitary condition and keeping quality of milk. The tests are intended to be carried out on samples collected for microbiological analysis. Microbiological standards for cow milk have been presented in below Table.

Microbiological Standards for Cow Milk

Methylene blue and Resazurin reduction tests measure the bio-chemical activity of microorganisms in milk. They get reduced, if added to milk, after a certain time and lose their colour. Quick reduction of a given quantity of dye means high microbiological activity and vice versa.

Half-hour methylene blue reduction (MBR) test: The length of time taken by milk to de-colourize methylene blue is a fairly good measure of its bacterial content.Ten ml milk in sterile test tube is well mixed with one ml methylene blue solution.Tube is closed with sterile rubber stopper and incubated in a water bath maintained at 37.5 + 0.5oC. Tube is inspected after 30 minutes and seen whether milk in the tube is de-colourized at least up to within 5 mm of the surface. Milk is graded as in above Table.

Ten-Minute resazurin test: Since resazurin reduction occurs in two stages, the first from blue to pink, and the second from pink to colourless, the quality of milk can be assessed easily in shorter time. Procedure is the same as described in MBR test. At the end of 10 minutes + 30 second of incubation, the tube is removed from the water bath and immediately the colour is matched with the resazurin disc in the comarator. The result shall be interpreted as follows:

 Direct Microscopic Count (DMC): The direct microscopic count method consists of examination of milk under a compound microscope. It enables the rapid estimation of the total bacterial population of a sample of milk and also reveals useful information for tracing the source of contamination in milk.